Ovens

ABSTRACT

A light weight, low thermal mass modular tunnel oven comprising two convection modules each having a top, bottom and side wall with hot gas manifolds mounted on each side wall for directing hot gas above and below a conveyor supported on the manifolds, and a radiant module following the second convection module and having top, bottom and side walls, a radiant heater being supported above the conveyor running through all the modules.

United States Patent [191 Sperring et al.

i111 3,830,624 [4s] Aug. 20, 1974 OVENS Inventors: Richard Lawrence Sperring, 175

Pine Gardens, Ruislip; Stephen Raymond Stansfield, Langford Green, London; Mohamed Hassan Kubba, 301 Landon Rd., Isleworth, all of England Filed: Mar. 7, 1973 Appl. No.: 338,881

Related U.S. Application Data Continuation of Ser. No. 121,366, March 5, 1971, abandoned.

Foreign Application Priority Data Mar. 6, 1970 Great Britain 10937/70 U.S. Cl. 432/145, 432/247 Int. Cl. F27b 9/14 Field of Search 165/58; 432/122, 144, 145, 432/247 References Cited UNITED STATES PATENTS 5/1937 Brizzolara 165/58 2,363,120 l1/l944 Drew 432/178 2,981,528 4/1961 Culp 432/145 3,272,156 9/1966 Gilgore et al, 432/145 3,314,666 4/1967 Gajardo 432/82 3,368,802 2/1968 Morgan et al. 432/247 3,591,152 7/1971 Mills, Jr. 432/247 Primary Examiner-.lohn J. Camby [57] ABSTRACT A light weight, low thermal mass modular tunnel oven comprising two convection modules each having a top, bottom and side wall with hot gas manifolds mounted on each side wall for directing hot gas above and below a conveyor supported on the manifolds, anda radiant module following the second convection modulev and having top, bottom and side walls, a radi` ant heater being supported above the conveyor running through all the modules.

25 Claims, 14 Drawing Figures PATENEnAuszolsM sum1-ur s LAM ATTORNEY ovENs This is a continuation of application Ser. No. 121,366, filed Mar. 5, 1971, now abandoned.

This invention is concerned with improvements in and relating to heat exchange apparatus particularly for use with edible products on an industrial scale. One such apparatus is a tunnel oven.

Heretofore, tunnel ovens have been constructed, for baking a stream of product, having a conveyor which is movable through a high thermal mass tunnel heated usually by gas or oil.

Such tunnels have proved satisfactory. However they are massive and therefore, when erected, are permanent units. Due to their high thermal mass they respond slowly to a change of heat input so that if a product at the output of the tunnel is not baked to the desired degree, a change of heat input to correct the condition will not be effective immediately the condition becomes apparent.

It is an object of the invention to provide an improved apparatus which obviates the above disadvantages.

According to the present invention, there is provided a heat exchange module comprising a body of low thermal mass forming a top wall, a bottom wall and side walls defining a tunnel open at both ends, a first manifold mounted on each side wall for connection to a source of heat exchange gas, mounting means on each manifold for supporting a conveyor for conveying articles through the tunnel and wherein each first manifold has at least one outlet for directing heat exchange gas therefrom transversely of the tunnel.

When used as an oven module the body is preferably formed of panels of light weight rigid sheet, such as Aeroweb, the insulation material is preferably Kao- Wool held to the panels by wire mesh or sheet metal panels. An oven preferably comprises three modules, a first module as above heated by hot gases pumped into the module primarily to the zone below the conveyor, the second as above by hot gases mainly fed to the zone above the conveyor and the third heated by a radiant heater situated above the conveyor. The first and second modules are generally described as convection modules.

Further features and advantages will become apparent from the following description of an embodiment thereof, given by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 is a section through a perspective view of a convection oven module in which details of the wall construction and conveyor have been omitted for clarit yF IG. 2 is a diagrammatic view of the heating gas circuit of the oven module of FIG. 1;

FIG. 3 is a section through a perspective view of a radiant heated oven module in which details of the wall construction and conveyor have been omitted for clarity;

FIG. 4 is a vertical section through the connection between a module side and top wall with the insulation omitted;

FIG. 5 is a vertical section through an alternative connection between a module side and top wall with the insulation omitted;

FIG. 6 is a vertical section through a module side and bottom wall connection with the insulation omitted; FIG. 7 is a horizontal section through the connection between two module walls with the insulation omitted;

FIG. 8 is a perspective view of the radiant heater used with the module of FIG. 3;

FIG. 9 is a perspective view of the heater of FIG. 8 during construction;

FIG. 10 is a vertical section through a conveyor system used in the modules of FIGS. 1 and 3;

FIG. 11 is a section on the line 11-11 of FIG. 10;

FIG.'12 is a plan view of the side portion of the conveyor system of FIG. 10 with part broken away;

FIG. 13 is a diagrammatic perspective view of the system of FIG. 10 with parts removed, and

FIG. 14 is a vertical section through an alternative conveyor system.

In the preferred embodiment the tunnel oven comprises three different modules. The first is shown in FIGS. 1 and 2 and comprises a tunnel made up of top, bottom and two side walls l, 2 and 3 respectively. Each wall comprises a light and rigid panel preferably of Aeroweb, being a metallic honeycomb structure with the cells extending between inner and outer metal or glass-fibre reinforced skins. To each panel is secured, by wire mesh or sheet metal 4 a heat insulating layer 5 of, for example, Kao-Wool. The mesh or sheet metal 4 may be held to the panels by screws (not shown).

The panels may be joined together by corner pieces 6 (FIG. 4), again of the same or similar light rigid material, attached, for example, by adhesive to the side pan els 3. These pieces 6 are secured to the top and bottom panels by mating extrusions 117, 118, female extrusion 117 being secured within the outer skins of the'corner piece and the extrusion V118 projecting from the edge I of the top wall panel l. Self-tapping bolts 119 secure the extrusions in mating relationship.

Preferably the top and side panels 1, 3 are joined by corner pieces 6a, FIG. 5, each corner piece being secured to the top panel 1 by mating extrusions l17a, l18a and bolts l19a, similar to extrusions 117, 118 and bolts 119, and to the side panels 3 by abutment between extrusions 120, 121. These abutted extrusions are held together by pop rivets 122 and adhesive. The corners may be completed by metal angle pieces 123 rivetted to the side panel 3 and corner pieces 6a respectively. The side and bottom panels 3, 2 are joined by corner pieces 6b, FIG. 6, connected to the bottom panel by mating extrusions 117b, 118b and bolts 119b and to the side panels 3 by abutted, adhered and rivetted extrusions 124, 125. The corners may be completed by metal angle members 126 rivetted to the side panels 3 and corner pieces 6b.

The first module is heated by circulated hot gas and to this end on each side wall of the tunnel is a manifold 10 having outlets l1 and l2, the pore size of grids covering the outlets 1l, l2 is larger for the outlets 12 than for outlets 1l so that the hot gas is discharged predominantly on to the lower surface of the conveyor 13 shown in FIG. 1 in broken lines. The manifolds 10 are supplied with hot gas by ducts 15 and the module is exhausted through inlets 16 of exhaust manifolds 17 coupled to return ducts 18.

The outlets 11 and inlets 16 of the supply and exhaust manifolds are staggered on each side of the tunnel and an outlet on one side is set opposite an inlet on the other side. The first module achieves heating up of products on the conveyor and constant rate mass transfer.

The gases may be circulated by a fan 19 (FIG. 2) having a fresh air inlet duct 20 (shown in broken lines), and an outlet coupled to a gas combustion chamber 21 of which the outlet is coupled to ducts 15. The gases are removed from the module by a fan 22 connected to ducts 18 by connectors 18', the output of the fan 22 being fed to the inlet duct of fan 19 and to an exhaust duct 22. Alternatively, the gases may be recirculated directly by the suction of fan 19, via duct 20.

Controlsl are provided for the module and these include an oven gas temperature responsive element up- 4'stream of the combustion chamber 21 to control the heat supplied in that chamber. A water vapour control may also be provided which will maintain a desired water vapour level in the gas stream by varying the quantity of gas discharged to the exhaust duct 22. The diversion may be effected using a conventional damper controlled by a water vapour sensor. Alternatively the diversion may be effected by causing the recycled gas to adhere to a curved surface when travelling toward the inlet duct 20 and to detach the gas stream, as by means of a gas jet, from that surface to thereby cause it to pass into the exhaust duct 22 A combustion product sensor may be also provided in the oven operable to divert recycled gases from the duct 20 to the exhaust outlet 22.

Preferably the air inlet duct 20 is omitted and the outlet duct of fan 22 is connected directly to the inlet duct of fan 19. A water vapour sensor in the oven is operable to divert recycled gases from the inlet duct of the fan 19 to the exhaust outlet duct 22 and to introduce fresh gas proportionately into the inlet duct of fan 19. This diversion is effected by means of an analog fluidic valve (not shown) utilizing the wall attachement principle. The valve gives a constant output to exhaust which is adjusted initially to be approximately equal to the mean level of the products of combustion which is superimposed on an analog output produced by an analog input of fresh gas into the control part of the valve. The analog input of fresh gas is controlled by the water vapor sensor. A combustion product control may be provided to override the water vapour control if a critical level of combustion products is reached.

To obtain a rapid product heating at the entry to the first module, a micro-wave heating unit may be provided adjacent to the inlet.

The second module is similar to the first being of like panel and heat insulation construction and is coupled to the first by male and female extrusions 218, 129 (FIG. 6), though in this case insteadrof self-tapping bolts, standard bolts 130 may be used.

The second module is again heated by combustion gases but these gases are fed primarily to the zone of the tunnel above the conveyor and are again extracted from above the conveyor. To this end the pore sizes of the grids convering the outlets 11, 12 is reversed so that hot gas is discharged predominantly from the outlets 11. The second module is throught to achieve the change of state of the product together with mass transfer. Controls may be provided on the second module similar to those of the first module.

The third module has no gas or air circulation means but includes a gas or electric radiant heater 27, below the insulation layer of the top panel. This will effect case hardening of the upper part of the product. Here a colour and/or temperature control may be provided by a colour and/or temperature responsive unit which will control heat input to the radiant heater. As shown in FIG. 3 the bottom panel 2 is omitted and a base with an insulation layer is provided immediately below the return flight of the conveyor. This module is connected to the second module by extrusion similar to those shown in FIG. 7.

The heater 27 may consist of a plurality of strips 27 suspended al short distance below a reflective metal plate (not shown) positioned between the strips and the insulation of the top panel. Each strip may consist of flat resistance elements sandwiched between mica or similar material, which is further enveloped with Kaowool paper. Preferably each heat strip 27 is an electrical radiant heater including an electrical heating element comprising electrically resistive material and an electrically insulating body in which the heating element is embedded, the body incorporating fibrous alumino-silicate material and a hardener applied in liquid form to the fibrous material during manufacture, so as to impregnate the fibrous material, and then caused or permitted to harden so as to give the body the desired rigidity.

The body of each heater strip 27, FIG. 8, comprises two sheets of hardened Triton Kao-Wool (Trade Name) 210, 212 between which a heating element is sandwiched with its ends 214 and 216 projecting to provide electrical terminals. v

The details of a heater strip are illustrated more clearly by FIG. 9 which is a view of part of the heater strip during an intermediate stage of construction, before the sheet 212 is added. The heating element cornprises a plurality of strands of coiled resistance wire 218 connected electrically in series, although of course a plurality of separate heating elements connected electrically in parallel could be used. The strands 218 are laid on the sheet 210 in grooves between parallel elongate ribs 220 formed by strips of Triton Kao-Wool laid on sheet 10 on the sheet 210. At the two ends of the ribs are disposed two pairs of insulating strips 222 224, and 226, 228, for example mica sheets. One end of each of the wires 218 is trapped between strips 222 and 224, with the terminals 214 and 216 projecting from the strips 222 and 224, and the other ends of the wires 218 are trapped between the strips 226 and 228. The strips are secured together, for example by bolts such as 230.

The sheet 212 is then positioned on and pressed down on to the assembly shown in FIG. 9 so that the Triton Kao-Wool, which is soft at this stage, is pressed into intimate contact with the wires 18. If dry Triton Kao-Wool has been used, the entire assembly is then impregnated with Triton Hardener (Trade Name). Lastly, the impregnated or pre-impregnated assembly is allowed to dry and harden.

:The material is electrically and thermally insulating, has a low thermal mass, high resistance to thermal shock, high emissivity and can be run continuously at a temperature of up to 1,260C. When the hardener is used, the material is rigid and self-supporting.

The conveyor 13 for the assembled modules is advantageously made so that it is removable and adaptable to varying lengths or numbers of modules. Preferably the conveyor 13 comprises a flexible belt 310 (FIGS. 10 to 13) of woven glass fibre material provided with a nonstick coating of PTFE. Female press studs 312 are secured to the belt, for example by rivetting, at intervals along each side of the belt. The material of the belt may be of open woven construction, in which case strips of reinforcing material (not shown) are secured to the side edges of the belt before the press studs are attached both to prevent the material tearing in the vicinity of the press studs and to prevent fraying. Alternatively, the belt may be close woven construction, not subject to fraying, in which the case patches of reinforcing material underneath each press study are suffic1ent.

A pair of chains 314 and 315 are provided, disposed at opposite sides of the belt, each chain comprising a series of rollers 316 each mounted for free rotation on a spindle 318, the spindle being linked on both sides of the rollers by overlapping side cheeks 320 and 322. At intervals corresponding to the press studs 312, the side cheeks are provided with flanges 324 to which plates 326 are rivetted, the plates 326 carrying male press studs 328 for co-operating with the female press studs 312.

The press stud fasteners and the chains may be made of metal. If the conveyor is to pass through a region in which products thereon are heated by microwave electromagnetic radiation or induction heating, it may be desirable to shield the press stud fasteners and chains from the radiation or the induction field. Advantageously at least the press stud fasteners, and the chains also, are made of dielectric material, such as nylon, so that it is unnecessary to shield them from the radiation or the induction field. The chains and press stud fasteners are arranged so that the centre line of the spindles 18 coincides with the centre line of the belt 10.

Referring now particularly to FIG. 13, the belt 310 runs between two rollers 327, mounted on spindles 329 each provided with a pair of sprocket wheels 331 and 333 mounted at opposite sides of the rollers 327 and engaging in the intervals between the rollers of the respective chains 314 and 315. One of the spindles 329 is driven through gearing by an electric motor (not shown), and the other may be movable in a direction parallel to the direction of motion of the belt 310 for adjusting the tension-in the belt. The rollers 327 preferably each consist of a light steel core covered with foam rubber.

The upper flight of the belt 310 slides on a flat plate 330 (FlG. 9) which is mounted on a further plate 332. The two plates 330 and 332 may be made of honeycomb material, or other perforate material, to facilitate circulation of air. The plate 330 supports the weight of articles on the conveyor. The plate 332 is supported on either side on the lower flange of two channel section members 338 mounted on the inner walls of the manifolds 10 in modules one and two and on conveyor bearing elements 33 in module three. The rollers 316 ofthe conveyor chains are guided between flanges 334, 336 extending respectively from the plate 332 and the upper flange of the respective channel section member 338.

y During assembly of the conveyor, the two chains 314 and 315 are passed along the track between the flanges 334 and 336 and over the sprocket wheels 331, 333, 335 and 337 located beyond the ends of the modules and joined on themselves to form endless loops. The belt 310 is then progressively secured to the chains by engaging the female press studs 312 with the male press studs 328. The belt is preferably of such a length that It will be appreciated that the belt 310 and chainsA 314, 315 can be readily removed from the oven modules. This is particularly convenient for cleaning purposes, for example. Moreover, the length of the conveyor can be altered for use with ovens comprising more or less modules, since corresponding sections can readily be removed from or added to the chains 314 and 315 and the belt 310 shortened in length or substituted by a shorter or longer belt.

The press studs could be replaced by other releasable fasteners, such as clips, and the chains 314 and 315 could be replaced by other flexible elements formed into endless loops and having apertures formed at intervals for engagement by the sprocket wheels 331, 333, 335 and 337. Indeed such apertures could be fonned in the belt 310 itself.

In a modification, referring to FIG. 14, the conveyor belt 430 is passed around two end rollers mounted outside the oven. Within the modules, the upper flight runs on Weld Mesh (Trade Mark) or similar bars 431 which extend between channels 332 on the inner walls of the manifold 10 in modules one and two and on conveyor bearing elements 433 (FIG. 3) in module three. The Weld Mesh has passages 434 therethrough to allow free circulation of gas. When an oven is to be dismantled or a module replaced for servicing, a roller may be removed and the conveyor band withdrawn through the tunnel, the bars 431 being removed successively as they are extracted from the channels.

It will be seen that the oven construction described has a low thermal mass and has therefore a fast response time, a quick to heat up and to vary in operation, is made up of basic light modules, is easily assembled and dismantled and is adaptable for different products. Furthermore, the tunnel described provides various modules of heating suited to the particular stages of a baking process.

While an oven made of three units has been described, the invention contemplates one or more of each type of module in a tunnel. For further adaptation to suit particular products, the grids in the manifold outlet and inlets may be changeable for varying the gas flow characteristics.

Additional controls may be utilized. For example a control may be provided responsive to water vapour to feed steam into the chamber of the first or second module and/or a control may be provided in those modules responsive to product moisture. This control may be responsive to a micro-wave moisture detector and may be applied to the gas circulation pump to increase the rate of gas flow per unit time in the oven.

We claim:

l. A heat exchange apparatus comprising:

a. a body of low thermal mass forming a top wall, a bottom wall and'side walls defining a tunnel open at both ends,

b. first manifolds located between said side walls,

c. ports in said manifolds for admitting gas to the zone of the tunnel between the manifolds,

d. means for varying the temperature of gas admitted into said zone,

e. first drive means for driving gas past said temperative varying means and through said manifolds,

f. outlet means positioned opposite said ports to permit flow of gas across said zone and from said tunnel,

g. second drive means for drawing gas from said zone through said outlet means and for driving that gas to the input os said first drive means and to exhaust means, and

h. conveying means for conveying atricles through said tunnel, said conveying means including a flexible endless conveying surface and supporting means therefor provided on each manifold.

2. Apparatus as claimed in claim 1 comprising second manifolds mounted between said side walls and connected with said outlet means and said second drive means..

3. Apparatus as claimed in claim l comprising control means for controlling the proportion of gas from said second drive means which is fed to the inlet of said first drive means and to said exhaust means.

4. Apparatus as claimed in claim 3 comprising means for feeding fresh gas to the inlet of said irst drive means at a rate which is substantially equal to the rate at which gas is fed to said exhaust means.

5. Apparatus as claimed in claim 3 wherein said control means includes a control valve connected between said first and second drive means.

6. Apparatus as claimed in claim 5 wherein said control valve is a fluidic valve which includes means for feeding fresh gas to the inlet of said first drive means.

7. Apparatus as claimed in claim 6 wherein said control means includes a water vapour detector in said zone and connected to control said valve.

8. Apparatus as claimed in claim l wherein said first and second drive means and said heating means are po sitioned within said tunnel.

9. Apparatus as claimed in claim 1 wherein said ports are positioned to admit gas into said zone both above and below said conveying surface, said ports being adapted to admit different amounts of gas above said conveying surface as below said conveying surface.

10. An oven comprising first and second modules each comprising:

a. a body of low thermal mass forming a top wall, a bottom wall and side walls defining a tunnel open at both ends,

b. first manifolds located between said side walls,

c. ports in said manifolds for admitting gas to the zone of the tunnel'between the manifolds,

d. heating means for gas admitted into said zone,

e. first drive means for driving gas past said heating means and through said manifolds,

f. outlet means positioned opposite said ports to permit flow of gas across said zone and from said tunnel,

g. second drive means for drawing gas from said zone through said outlet means and for driving that gas to the input of said first drive means and to exhaust means, and

h. conveying means for conveying articles through said tunnel, said conveying means including a flexible endless conveying surface and supporting means therefor provided on each manifold, said oven also including means provided on said first and second modules for detachably connecting an open end of one module to an open end of the other module and wherein said conveying surface is common to both modules and said portsv of each said manifold are positioned to admit gas both above and below said conveying surface and are adapted such that gas will be admitted predominantly below said conveying surface in said first module and predominately above said conveying surface in said second module.

1l. An oven as claimed in claim l0 wherein each module comprises control means for controlling the proportion of gas fed to said respective exhaust means and to the input of said respective first drive means.

12. An oven as claimed in claim l1 wherein each control means includes a water vapour detector in said respective zone and connected to control valve means between said respective first and second drive means.

13. An oven as claimed in claim 12 comprising a third module comprising:

a. a low thermal mass body forming a top wall, a bottom wall and side walls defining a tunnel open at both ends,

b. conveying means for conveying articles through said tunnel, said conveying means comprising a flexible endless conveying surface and supporting means therefor, and

c. a radiant heater situated above said conveying surface, wherein said conveying surface is common to said first, second and third modules and means are provided on said second and third modules for detachably connecting an open end of said third module to that open end of said second module remote from said first module.

14. Apparatus as claimed in claim 13 wherein said radiant heater comprises a plurality of heater strips, each strip comprising an electrical heating element of electrically resistive material and an electrically insulating body in which said heating element is embedded, said body incorporating fibrous alumino-silicate material.

15. An oven as claimed in claim l wherein said conveying surface is a belt of polytetrafluoroethylene coated woven material and locating means are provided on the lateral margins of the belt for locating the belt relative to said supporting means against transverse movement.

16. Apparatus as claimed in claim l5 wherein said locating means engage in said supporting means and are detachably provided on the lateral margins of said belt.

17. Apparatus as claimed in claim l wherein said body of said first module comprises panels of a light weight metal honeycomb structure, the cells of which extend between metal skins, vthe panel being covered on the inner surfaces therein with heat insulation material.

18. An oven as claimed in claim 10 wherein said body of each of said first and second modules comprises panels of a light weight metal honeycomb structure, the cells of which extend between metal skins, the panels being covered on the inner surfaces thereof with heat insulation material.

19. An oven as claimed in claim 13 wherein said body of said third module comprises panels of a lightweight metal honeycomb structure, the cells of which extend between metal skins, the panels being covered on the inner surfaces thereof with heat insulation material.

20. An oven as claimed in claim 13 comprising a plurality of at least one of the first, second and third mdoules.

2l. An oven comprising a first module comprising:

a. a body of low thermal mass forming a top wall, a bottom wall and side walls defining a tunnel open at both ends,

b. first manifolds located between said side walls,

c. ports in said manifolds for admitting gas to the zone of the tunnel between the manifolds,

d. heating means for gas admitted into said zone,

e. first drive means for driving gas past said heating means and through said manifolds,

f. outlet means positioned opposite said ports to permit flow of gas across said zone and from said tunnel,

g. second drive means for drawing gas from said zone through said outlet means and for driving that gas to the input of said first drive means and to exhaust means, and

h. conveying means for conveying articles through said tunnel, said conveying means including a flexible endless conveying surface and supporting means therefor provided on each manifold, and a second module comprising:

i. a low thermal mass body forming a top wall, a bottom wall and side walls defining a tunnel open at both ends,

ii. conveying means for conveying articles through said tunnel, said conveying surface and supporting means therefor, and

iii. a radiant heater situated above said conveying surface, wherein said conveying surface is common to said first and second modules and means are provided on said first and second modules for detachably connecting an open end of one module to an open end of the other module.

22. Apparatus as claimed in claim 1 wherein said temperative varying means is a heating means.

23. Apparatus as claimed in claim 4 wherein said temperative varying means is a heating means.

24. Apparatus as claimed in claim 7 wherein said temperative varying means is a heating means.

25. Apparatus as claimed in claim 9 wherein said temperative varying means is a heating means. 

1. A heat exchange apparatus comprising: a. a body of low thermal mass forming a top wall, a bottom wall and side walls defining a tunnel open at both ends, b. first manifolds located between said side walls, c. ports in said manifolds for admitting gas to the zone of the tunnel between the manifolds, d. means for varying the temperature of gas admitted into said zone, e. first drive means for driving gas past said temperative varying means and through said manifolds, f. outlet means positioned opposite said ports to permit flow of gas across said zone and from said tunnel, g. second drive means for drawing gas from said zone through said outlet means and for driving that gas to the input os said first drive means and to exhaust means, and h. conveying means for conveying atricles through said tunnel, said conveying means including a flexible endless conveying surface and supporting means therefor provided on each manifold.
 2. Apparatus as claimed in claim 1 comprising second manifolds mounted between said side walls and connected with said outlet means and said second drive means.
 3. Apparatus as claimed in claim 1 comprising control means for controlling the proportion of gas from said second drive means which is fed to the inlet of said first drive means and to said exhaust means.
 4. Apparatus as claimed in claim 3 comprising means for feeding fresh gas to the inlet of said first drive means at a rate which is substantially equal to the rate at which gas is fed to said exhaust means.
 5. Apparatus as claimed in claim 3 wherein said control means includes a control valve connected between said first and second drive means.
 6. Apparatus as claimed in claim 5 wherein said control valve is a fluidic valve which includes means for feeding fresh gas to the inlet of said first drive means.
 7. Apparatus as claimed in claim 6 wherein said control means includes a water vapour detector in said zone and connected to control said valve.
 8. Apparatus as claimed in claim 1 wherein said first and second drive means and said heating means are positioned within said tunnel.
 9. Apparatus as claimed in claim 1 wherein said ports are positioned to admit gas into said zone both above and below said conveying surface, said ports being adapted to admit different amounts of gas above said conveying surface as below said conveying surface.
 10. An oven comprising first and second modules each comprising: a. a body of low thermal mass forming a top wall, a bottom wall and side walls defining a tunnel open at both ends, b. first manifolds located between said side walls, c. ports in said manifolds for admitting gas to the zone of the tunnel between the manifolds, d. heating means for gas admitted into said zone, e. first drive means for driving gas past said heating means and through said manifolds, f. outlet means positioned opposite said ports to permit flow of gas across said zone and from said tunnel, g. second drive means for drawing gas from said zone through said outlet means and for driving that gas to the inpUt of said first drive means and to exhaust means, and h. conveying means for conveying articles through said tunnel, said conveying means including a flexible endless conveying surface and supporting means therefor provided on each manifold, said oven also including means provided on said first and second modules for detachably connecting an open end of one module to an open end of the other module and wherein said conveying surface is common to both modules and said ports of each said manifold are positioned to admit gas both above and below said conveying surface and are adapted such that gas will be admitted predominantly below said conveying surface in said first module and predominately above said conveying surface in said second module.
 11. An oven as claimed in claim 10 wherein each module comprises control means for controlling the proportion of gas fed to said respective exhaust means and to the input of said respective first drive means.
 12. An oven as claimed in claim 11 wherein each control means includes a water vapour detector in said respective zone and connected to control valve means between said respective first and second drive means.
 13. An oven as claimed in claim 12 comprising a third module comprising: a. a low thermal mass body forming a top wall, a bottom wall and side walls defining a tunnel open at both ends, b. conveying means for conveying articles through said tunnel, said conveying means comprising a flexible endless conveying surface and supporting means therefor, and c. a radiant heater situated above said conveying surface, wherein said conveying surface is common to said first, second and third modules and means are provided on said second and third modules for detachably connecting an open end of said third module to that open end of said second module remote from said first module.
 14. Apparatus as claimed in claim 13 wherein said radiant heater comprises a plurality of heater strips, each strip comprising an electrical heating element of electrically resistive material and an electrically insulating body in which said heating element is embedded, said body incorporating fibrous alumino-silicate material.
 15. An oven as claimed in claim 1 wherein said conveying surface is a belt of polytetrafluoroethylene coated woven material and locating means are provided on the lateral margins of the belt for locating the belt relative to said supporting means against transverse movement.
 16. Apparatus as claimed in claim 15 wherein said locating means engage in said supporting means and are detachably provided on the lateral margins of said belt.
 17. Apparatus as claimed in claim 1 wherein said body of said first module comprises panels of a light weight metal honeycomb structure, the cells of which extend between metal skins, the panel being covered on the inner surfaces therein with heat insulation material.
 18. An oven as claimed in claim 10 wherein said body of each of said first and second modules comprises panels of a light weight metal honeycomb structure, the cells of which extend between metal skins, the panels being covered on the inner surfaces thereof with heat insulation material.
 19. An oven as claimed in claim 13 wherein said body of said third module comprises panels of a lightweight metal honeycomb structure, the cells of which extend between metal skins, the panels being covered on the inner surfaces thereof with heat insulation material.
 20. An oven as claimed in claim 13 comprising a plurality of at least one of the first, second and third mdoules.
 21. An oven comprising a first module comprising: a. a body of low thermal mass forming a top wall, a bottom wall and side walls defining a tunnel open at both ends, b. first manifolds located between said side walls, c. ports in said manifolds for admitting gas to the zone of the tunnel between the manifolds, d. heating means for gas admitted into said zone, e. first drive means for driving gas past sAid heating means and through said manifolds, f. outlet means positioned opposite said ports to permit flow of gas across said zone and from said tunnel, g. second drive means for drawing gas from said zone through said outlet means and for driving that gas to the input of said first drive means and to exhaust means, and h. conveying means for conveying articles through said tunnel, said conveying means including a flexible endless conveying surface and supporting means therefor provided on each manifold, and a second module comprising: i. a low thermal mass body forming a top wall, a bottom wall and side walls defining a tunnel open at both ends, ii. conveying means for conveying articles through said tunnel, said conveying surface and supporting means therefor, and iii. a radiant heater situated above said conveying surface, wherein said conveying surface is common to said first and second modules and means are provided on said first and second modules for detachably connecting an open end of one module to an open end of the other module.
 22. Apparatus as claimed in claim 1 wherein said temperative varying means is a heating means.
 23. Apparatus as claimed in claim 4 wherein said temperative varying means is a heating means.
 24. Apparatus as claimed in claim 7 wherein said temperative varying means is a heating means.
 25. Apparatus as claimed in claim 9 wherein said temperative varying means is a heating means. 